Uninterruptible power supply with improved output regulation

Abstract

A microprocessor based uninterruptible power supply generates an AC electrical output power from an AC input source when the AC input source is within settable voltage limits and from an auxiliary source of DC power when the AC input source is outside said the voltage limits. The microprocessor provides monitoring of the AC input voltage to determine its amplitude and frequency, and uses this information to set the magnitude and frequency of the AC output voltage and to select the source of the AC output power. A power factor improvement circuit boosts the selected source to a high DC voltage which is inverted by a chopper circuit to produce a high frequency AC voltage is rectified and sent to a PWM inverter to produce the AC output power. An average current feedback loop provides the output voltage regulation and utilizes current limiting to provide output current limiting for protection against short circuits and overloads. A voltage phase relationship is maintained between the AC input power and AC output power to eliminate voltage and current transients at transfer times between the AC and DC input sources. A serial data communications port sends data sent over a network that includes signals indicative of the status of the uninterruptible power supply and receives data from the network for controlling operation of the uninterruptible power supply.

I claim: 
     
       1. An improved uninterruptible power supply for supplying a load with AC electrical output power from an AC input source when said AC input source is within settable voltage limits, and supplying said load with AC electrical output power from a DC auxiliary power source when said AC input source is outside said voltage limits, said improved uninterruptible power supply comprising: A. means for monitoring said AC input source to determine frequency and amplitude of said AC input source;   B. means for converting said AC input source to a first DC voltage;   C. means for transferring between said AC input source and said DC auxiliary power source to select an input source of said AC electrical output power, said input source being said first DC voltage when the amplitude of said AC input source is within said voltage limits and said DC auxiliary power source when the amplitude of said AC input source is outside said voltage limits;   D. means for providing active power factor correction of said AC input source when said AC input source is said input source;   E. means for boosting and regulating said input source to produce a second and higher DC voltage;   F. means for converting said second DC voltage to produce equal and commonly grounded positive and negative DC voltages;   G. means for inverting said positive and negative DC voltages to produce said AC electrical output power;   H. means for generating a signal proportional to an average AC electrical output current of said AC; electrical output power to provide an average current feedback loop to said inverting means:   I. means for regulating said AC electrical output power to a voltage amplitude corresponding to said determined frequency;   J. means for limiting said AC electrical output power to a current level within settable current limits to prevent failure of said inverting means; and   K. means for maintaining a voltage phase relationship between said AC input power and said AC output power during said transferring between said AC input power source and said DC auxiliary power source.   
     
     
       2. The improved uninterruptible power supply of claim 1 in which said conversion means of said AC input source to a first DC voltage is a bridge rectifier and further includes means for filtering said AC input source to remove electromagnetic and radio interference. 
     
     
       3. The improved uninterruptible power supply of claim 1 in which said transferring means includes a means for isolating said AC input source from said DC auxiliary power source when said DC auxiliary power source is the source of said AC electrical output power. 
     
     
       4. The improved uninterruptible power supply of claim 1 in which said conversion means of said second DC voltage to produce equal and commonly grounded positive and negative DC voltages includes a chopper circuit for producing pulse width modulated (PWM) driver signals to control conduction of a four quadrant driver circuit, said four quadrant driver circuit to provide controlled switching of said second DC voltage to produce an equivalent AC sinusoidal voltage at a frequency substantially higher than said determined frequency of said AC input source. 
     
     
       5. The equivalent AC sinusoidal voltage of claim 4 further coupled to the primary of an isolation transformer to produce said positive and negative DC voltages by rectifying means of center-tapped secondaries of said isolation transformer. 
     
     
       6. The improved uninterruptible power supply of claim 1 in which said inverting means of said positive and negative DC voltages includes a PWM inverter to produce said AC electrical output power, said PWM inverter including means for comparing the voltage amplitude of said AC electrical output power to the voltage amplitude corresponding to said determined frequency to produce a voltage error signal, means for comparing said signal proportional to average AC electrical output current with said voltage error signal to produce a current error signal, said current error signal for modulating the width of pulses used to control conduction of output driver circuits in said inverting means. 
     
     
       7. The improved uninterruptible power supply of claim 6 in which said PWM inverter further includes means for limiting the magnitude of said voltage error signal, said limiting means to further limit the current error signal and thus, limit the width of the pulses used to control conduction of said output driver circuits and limit the AC electrical output power to a current level below that level that could cause failure of said output driver circuits. 
     
     
       8. The improved uninterruptible power supply of claim 1 further including means for comparing the voltage phase relationship of said AC input power source with said AC electrical output power to produce a voltage reference signal, said voltage reference signal for controlling operation of said means for inverting said positive and negative DC voltages to produce said AC electrical output power in voltage phase synchronization with said AC input power source. 
     
     
       9. The improved uninterruptible power supply of claim 1 wherein said DC auxiliary power source is a battery. 
     
     
       10. The improved uninterruptible power supply of claim 9 including means for monitoring the voltage level of said battery and means for charging said battery to maintain a full electrical charge on said battery when said AC input source has been selected as the source of said AC electrical output power, said charging means for charging said battery at one rate when said battery is not fully charged and at a lower rate when said battery is near full charge. 
     
     
       11. The improved uninterruptible power supply of claim 1 further including a microprocessor, said microprocessor providing voltage and current monitoring functions of said AC input power source, said DC auxiliary power source, and said AC electrical output power, said monitoring functions controlling operation of said uninterruptible power supply. 
     
     
       12. The microprocessor of claim 11 further including means for monitoring temperature rises of output driver circuits used in said inverting means to produce said AC electrical output power, said temperature monitoring means preventing operation of said uninterruptible power supply when said temperature rises could result in failure of said output driver circuits. 
     
     
       13. The microprocessor of claim 11 further functioning as said monitoring means of said AC input source to determine frequency of said AC input source and including means for generating a voltage reference signal in phase with and of the same frequency as said AC input source, said voltage reference signal to maintain said AC electrical output power at the same frequency as said AC input source when said DC auxiliary power source is the source of said AC electrical output power. 
     
     
       14. The improved uninterruptible power supply of claim 1 further including means for connection to a serial communication network, said connection for sending and receiving data over said network. 
     
     
       15. The improved uninterruptible power supply of claim 14 wherein said data sent over said network includes signals indicative of the status of said uninterruptible power supply. 
     
     
       16. The improved uninterruptible power supply of claim 14 wherein said data received over said network includes signals means for controlling operation of said uninterruptible power supply. 
     
     
       17. In an uninterruptible power supply for supplying a load with AC electrical output power, a method for generating said AC electrical output power from an AC input source when said AC input source is within settable voltage limits and for generating said AC electrical output power from a DC auxiliary power source when said AC input source is outside said voltage limits, said method comprising: A. monitoring said AC input source to determine frequency and amplitude of said AC input source;   B. converting said AC input source to a first DC voltage;   C. transferring between said AC input source and said DC auxiliary power source to select an input source of said AC electrical output power, said input source being said first DC voltage when the amplitude of said AC input source is within said voltage limits and said DC auxiliary power source when the amplitude of said AC input source is outside said voltage limits;   D. providing active power factor correction of said AC input source when said AC input source is said input source;   E. boosting and regulating said input source to produce a second and higher DC voltage;   F. converting said second DC voltage to produce equal and commonly grounded positive and negative DC voltages;   G. inverting said positive and negative DC voltages to produce said AC electrical output power;   H. generating a signal proportional to an average AC output current of said AC electrical output power to provide an average current feedback loop for regulating said AC electrical output power to a voltage amplitude corresponding to said determined frequency;   I. limiting said AC electrical output power to a current level within settable current limits to prevent failure of said inverting means; and   J. maintaining a voltage phase relationship between said AC input power and said AC output power during said transferring between said AC input power source and said DC auxiliary power source and vice versa to prevent voltage and current transients at the time of said transferring.   
     
     
       18. The method of claim 17 wherein said DC auxiliary power source is a battery, said method further including monitoring the voltage level of said battery and charging said battery to maintain a full electrical charge on said battery when said AC input source has been selected as the source of said AC electrical output power, said charging of said battery at one rate when said battery is not fully charged and at a lower rate when said battery is near full charge. 
     
     
       19. The method of claim 17 further including exchanging data over a serial data communications network wherein said data sent over said network includes signals indicative of the status of said uninterruptible power supply and said data received over said network includes signals for controlling operation of said uninterruptible power supply. 
     
     
       20. The method of claim 17 wherein said uninterruptible power supply includes a microprocessor for sequencing operation of said uninterruptible power supply. 
     
     
       21. The method of claim 20 wherein said microprocessor performs said monitoring of said AC input source to determine frequency and amplitude of said AC input source, provides control signals to allow said transferring between said AC input source and said DC auxiliary power source as the source of said AC electrical output power based on said determined amplitude, and generates a sine reference signal for said maintaining of said voltage phase relationship between said AC input power and said AC output power based on said determined frequency.